1. Field of the Invention
The present invention relates to non-destructive evaluation of magnetizable materials, and in particular, to the non-destructive evaluation of stress in ferromagnetic materials.
2. Problems in the Art
The relationship between certain magnetic properties and certain characteristics of magnetizable materials is known in the art. For some 40 years or so, investigations into magnetization measurements as they relate to the structure of magnetizable specimens have occurred. It then became well known to some that relationships exist between magnetic properties and strain or stress in these materials.
Although various generalized correlations have been derived between individual magnetic properties and stress in magnetizable materials, a complete and practical understanding has not been achieved. This is particularly true of ferromagnetic materials.
The advantages and benefits of such an understanding would be significant. It would allow superior non-destructive evaluation (NDE) of materials such as steel, which is so pervasively used in our society. There is a real need to achieve the ability to non-destructively and efficiently derive information regarding stress in such materials. Advances in understanding the effect of stress on different materials would allow valuable insight into how materials behave under stress and how to evaluate stress. An example would be the non-destructive evaluation of construction steels.
Most current evaluation techniques are either destructive, or utilize non-destructive evaluation methods such as x-ray diffraction or ultrasound. The disadvantages of destructive evaluation are obvious.
X-ray diffraction requires costly and complex equipment, and a significant amount of time. It is also generally limited to deriving information from little more than surface layers of the material under analysis.
Ultrasound is being shown to be a valuable NDE tool, but it has limitations also. For example, material variables such as preferred grain orientation, composition, prior magnetic history (which is significant in steels), and inhomogeneity, can effect or even mask results.
As stated, while there has been significant work done investigating the relationship of magnetization properties with stress, it has not been fully understood or developed. The relationships are very complex. Most activity has been done with respect to applied stress (sometimes referred to as elastic strain) on materials. Relatively little work has been done on residual stress (sometimes referred to as plastic deformation), and there is even less understanding regarding the relationships in that area.
As is well known, the magnetization of a magnetizable material can be represented by a plot of flux density B in the material versus a varying applied magnetic field of intensity H. What are referred to as the anhysteretic magnetization curve and hysteresis magnetization curve, plotted in terms of B versus H, are also well known in the art. See for example, R.M. Bozorth, "Ferromagnetism", Van Nostrand, New York, 1951, which is incorporated by reference herein. See particularly pages 8-9, Chapter 11, pages 507, 512, and 548-549. These curves vary from material to material. Also, as has been previously discussed, stress can affect these curves.
All magnetizable materials have properties which can be characterized as giving each particular material its own magnetic "signature". Many of these properties are related to the anhysteretic and hysteresis magnetization curves. Examples of these properties are coercivity, hysteresis loss, initial permeability and susceptibility, remanence, hysteretic permeability and susceptibility, differential susceptibility, and maximum differential permeability and susceptibility. All of these properties are well known to those skilled in the art. It is also well known that susceptibility can be derived by subtracting unity from permeability.
It has been found that these properties are related not only to the microstructure of ferromagnetic materials, but also to the mechanical treatment of the material. That is, stress or strain, whether applied or residual, can effect these properties.
However, there is yet to be an understanding of the meaning of any of the general relationships between stress and those properties to be useful in practical application.
Even though it is observed that the anhysteretic and hysteresis curves, and correspondingly many of the magnetic parameters, vary with stress in the material, the observed changes are complex. Relationships have not been able to be reliably articulated.
Therefore, it is a principal object of the present invention to improve over or solve the deficiencies and problems in the art.
A further object of the present invention is to provide a method for evaluation of stress in ferromagnetic materials from hysteresis curves and anhysteresis curves which provides a practical understanding of these relationships sufficient to allow useful evaluation of stress.
Another object of the present invention is to provide a method as above described which presents a simple expression for the variation of anhysteretic differential susceptibility at the origin (defined by M=0, H=0) with stress.
A still further object of the present invention is to provide a method as above described which is useful to utilize magnetization measurements to derive information regarding stress of a ferromagnetic material.
Another object of the present invention is to provide a method as above described which is useful with regard to ferromagnetic materials having residual stress, as well as applied stress.
These and other objects, features, and advantages of the present invention will become more apparent with reference to the accompanying specification and claims.